Subterranean “sliding” drilling operation typically involves rotating a drill bit on a downhole motor at the remote end of a drill pipe string. Drilling fluid forced through the drill pipe rotates the motor and bit. The assembly is directed or “steered” from a vertical drill path in any number of directions, allowing the operator to guide the wellbore to desired underground locations. For example, to recover an underground hydrocarbon deposit, the operator may drill a vertical well to a point above the reservoir and then steer the wellbore to drill a deflected or “directional” well that penetrates the deposit. The well may pass horizontally through the deposit. Friction between the drill string and the bore generally increases as a function of the horizontal component of the bore, and slows drilling by reducing the force that pushes the bit into new formations.
Such directional drilling requires accurate orientation of a bent segment of the downhole motor that drives the bit. Rotating the drill string changes the orientation of the bent segment and the toolface. To effectively steer the assembly, the operator must first determine the current toolface orientation, such as via measurement-while-drilling (MWD) apparatus. Thereafter, if the drilling direction needs adjustment, the operator must rotate the drill string to change the toolface orientation.
If no friction acts on the drill string, such as when the drill string is very short and/or oriented in a substantially vertical bore, rotating the drill string may correspondingly rotate the bit. Where the drill string is increasingly horizontal and substantial friction exists between the drill string and the bore, however, the drill string may require several rotations at the surface to overcome the friction before rotation at the surface translates to rotation of the bit.
Conventionally, such toolface orientation requires the operator to manipulate the drawworks brake, and rotate the rotary table or top drive quill to find the precise combinations of hook load, mud motor differential pressure, and drill string torque, to position the toolface properly. Each adjustment has different effects on the toolface orientation, and each must be considered in combination with other drilling requirements to drill the hole. Thus, reorienting the toolface in a bore is very complex, labor intensive, and often inaccurate.